A refraction factor of light varies according to a wavelength. Light of a shorter wavelength causes great refraction, and light of a longer wavelength causes small refraction. This induces a phenomenon, called chromatic aberration, of a position (hereinafter simply called an “image formation position”), where an image is formed, being changed by the wavelength of light.
For instance, in the case of a beam entering a lens from one point on an optical axis, an image formation position varies according to the wavelength of the beam. When a position where a green image is formed is taken as P, blue (B) light whose wavelength is shorter than that of green light forms an image at a position closer to the lens than P. Red (R) light, whose wavelength is longer than that of green light, forms an image at a position more distant from the lens than P. As mentioned above, aberration of an image formation position on the optical axis varying according to a wavelength is called axial chromatic aberration. When a position, where green light forms an image, is taken as a reference point on an image formation plane, a B image and an R image are out of focus as compared with the G image and become inferior in sharpness. When the focus position of the lens is adjusted so as to cause G light to come into focus, brightness of G light sharply changes stepwise at an edge, whilst the brightness of R light and that of B light change smoothly. As mentioned above, the brightness distributions of R, G, and B vary because of the principle of an overlap among point spread distributions. The greater the blur in an image on the image formation plane, the less clear a boundary area of the edge of the image. Hence, the image exhibits a smooth change. Accordingly, when a subject whose edge exhibits such a brightness distribution is photographed, a color is not accurately reproduced around the edge, thereby generating a false color. When an oblique beam from outside of the optical axis has entered the lens, the size of an image varies according to a wavelength, as does an image formation position. Such aberration is called chromatic aberration of magnification. By means of chromatic aberration of magnification, focus is not attained at the same position, even on the image formation plane. As a result, an image similarly becomes inferior in sharpness. In order to suppress such chromatic aberration, there is a method for previously measuring and storing, in memory, data pertaining to chromatic aberration of magnification of a lens; and performing corrections by reference to the data during photographing operation.
Japanese Patent Publication 2003-060983 discloses a technique for, in order to correct chromatic aberration of a lens when a high-brightness portion of the image signal is saturated, estimating original brightness by use of another image signal captured at an exposure level that at which an image signal to be corrected is captured—in relation to a signal portion having saturated brightness; and correcting chromatic aberration on the basis of the estimated brightness. Chromatic aberration is corrected by means of utilizing the magnitude (absolute value) of a G signal, which has passed through a high-pass filter, for gain control with respect to color-difference signals R-Y and B-Y, both being achieved in the edge portion, such that the gain of the color difference signals is reduced when the absolute value of the G signal is large.
Japanese Patent Publication 2005-136917 describes correction processing for diminishing visibility of an image component corresponding to non-image formation light in order to eliminate a flare from an optical diffraction element.
However, under the method for previously storing in memory data pertaining to chromatic aberration of magnification of a lens, an increase in memory capacity results in an increase in the number of adjustment processes. In consideration of the influence of axial chromatic aberration, there is a problem of difficulty being encountered in performing accurate corrections.
Further, although the technique described in Japanese Patent Publication 2003-060983 does not require previous storage of data for correction purpose into memory, a necessity for performing exposure twice will arise.